Composite of aluminum and boron nitride nanotubes and method for manufacturing same

ABSTRACT

There is provided a composite of a metallic matrix and boron nitride nanotubes, the metallic matrix including aluminum or an aluminum alloy. Also, there is provided a method for manufacturing the composite. The method includes: a powder mixing step of mixing a powder of boron nitride nanotubes and a powder of an element soluble in a molten metal of the metallic matrix to prepare a powder mixture of boron nitride nanotubes and a metallic matrix-soluble element; an alloy melt mixing step of mixing the powder mixture and the molten metal of the metallic matrix to prepare a metallic matrix melt mixed with boron nitride nanotubes; and a casting step of solidifying the metallic matrix melt mixed with boron nitride nanotubes to obtain the composite.

CLAIM OF PRIORITY

The present application claims priority from Japanese patent applicationserial no. 2018-078482 filed on Apr. 16, 2018, the content of which ishereby incorporated by reference into this application.

FIELD OF THE INVENTION

The present invention relates to technology of composites of metals andfine fibrous substances, and in particular, to a composite of a matrixof aluminum or an aluminum alloy (hereinafter referred to simply asaluminum) and boron nitride nanotubes dispersed in the matrix(hereinafter referred to as aluminum and boron nitride nanotubecomposite) and a method for manufacturing the composite.

DESCRIPTION OF RELATED ART

In order to improve mechanical properties of metallic materials,research and development has been carried out on technology of adding afine fibrous substance to or mixing it into a metallic matrix. Forexample, JP 2010-196098 A discloses a metal matrix composite obtained byimpregnating a preform including a plurality of fibers of a fibrousmaterial entangled in a three-dimensional space with a molten metal andsolidifying the metal and preform. In space defined by the fibers aretrapped metal powder particles with a carbon nanomaterial (carbonnanotubes or carbon nanofibers) attached on the surface of the particlesor incorporated in the particles. The metal powder and the molten metalare an aluminum alloy or a magnesium alloy.

On the other hand, in recent years, attention has been directed towardboron nitride nanotubes as a fine fibrous substance. Boron nitridenanotubes (hereinafter referred to as BNNTs as well) are nanotubes(NTs), which are cylinders formed of a sheet of alternately bondednitrogen (N) atoms and boron (B) atoms. BNNTs are considered to havemechanical properties comparable to those of carbon nanotubes (CNTs),which are cylinders formed of a sheet of carbon (C) atoms bonded witheach other, and have high thermal stability.

For example, Yanming Xue et al. (Materials and Design 88 (2015) 451-460)discloses a study in which an aluminum and boron nitride nanotubecomposite (hereinafter referred to as Al/BNNT composite) was fabricatedby a high-pressure torsion technique. In the study, a mixture of analuminum (Al) powder and boron nitride nanotubes (BNNTs) was subjectedto a torsion process under a high pressure of 5 GPa.

According to JP 2010-196098 A, there can be provided a metal matrixcomposite with excellent lubricity, cohesion resistance, wearresistance, and thermal conductivity. At present, however, new problemsare arising, such as poor interfacial bondability between the aluminumMatrix and the carbon nanomaterial, and insufficient homogeneousdispersion and insufficient chemical stability of the carbonnanomaterial in the aluminum matrix.

According to Yanming Xue et al, the obtained Al/BNNT composite has anamorphous ultra-thin Al—(BNO) layer (2-5 nm in thickness) at theinterface region between the Al and BNNTs and exhibits an improvedtensile strength of up to 420 MPa at room temperature, which is morethan double the tensile strength of pure Al materials. This suggests thepossibility of improvement of mechanical properties by combining Al andBNNTs to form a composite, which is enticing. However, since thetechnique disclosed in Yanming Xue et al. employs a specialmanufacturing method, a high-pressure torsion technique, it isdisadvantageous in that the composite has poor shape flexibility andshape controllability, and the cost of forming the composite into adesired shape is prone to be high. For new materials such as Al/BNNTcomposites to find practical applications (in particular, to replaceexisting materials), they have to be made available at low cost, as amatter of first priority. If Al/BNNT composites can be manufactured by acasting process, their disadvantage in shape flexibility and shapecontrollability can be overcome and their manufacturing costs can besignificantly reduced. Meanwhile, casting is considered as unsuitablefor manufacturing Al/CNT composites because CNTs react chemically withan Al melt to produce compounds such as carbides.

SUMMARY OF THE INVENTION

In view of foregoing, it is an objective of the present invention toprovide an Al/BNNT composite which has excellent shape flexibility andshape controllability and is capable of cost reduction. Also, anotherobjective of the invention is to provide a method for manufacturing theAl/BNNT composite.

According to one aspect of the invention, there is provided a compositeof a metallic matrix and boron nitride nanotubes, the metallic matrixincluding aluminum or an aluminum alloy. The boron nitride nanotubes aredispersed in the metallic matrix, and the metallic matrix is prepared bya melt-solidification process (has a melt-solidified structure).

In the above aspect of a composite (I) of the invention, the followingmodifications and changes can be made.

(i) The aluminum alloy may include aluminum as a main component and atleast one of silicon, copper, magnesium, and nickel.

According to another aspect of the invention, there is provided a methodfor manufacturing a composite of a metallic matrix and boron nitridenanotubes, the metallic matrix including aluminum or an aluminum alloy.The method includes:

a powder mixing step of mixing a powder of boron nitride nanotubes and apowder of an element soluble in a molten metal of the metallic matrix toprepare a powder mixture of boron nitride nanotubes and a metallicmatrix-soluble element;

an alloy melt mixing step of mixing the powder mixture and the moltenmetal of the metallic matrix to prepare a metallic matrix melt mixedwith boron nitride nanotubes; and

a casting step of solidifying the metallic matrix melt mixed with boronnitride nanotubes to obtain the composite.

Meanwhile, in the present invention, the molten metal of the metallicmatrix in the alloy melt mixing step may be a pure aluminum melt or analuminum alloy melt.

In the above aspect of a method for manufacturing a composite of ametallic matrix and boron nitride nanotubes (II) of the invention, thefollowing modifications and changes can be made.

(ii) The powder of an element soluble in a molten metal of the metallicmatrix may be a powder of silicon.

(iii) A ratio between the specific surface area of the powder of boronnitride nanotubes and the specific surface area of the powder of anelement soluble in a molten metal of the metallic matrix may be lessthan 10.

(iv) A mass ratio between the powder of boron nitride nanotubes and thepowder of an element soluble in a molten metal of the metallic matrixmay be equal to or more than 1:2 and equal to or less than 2:1.

(v) The aluminum alloy may include aluminum as a main component and atleast one of silicon, copper, magnesium, and nickel.

(vi) The powder mixing step may include:

a boron nitride nanotube suspension preparation substep of mixing thepowder of boron nitride nanotubes and an organic solvent to prepare aboron nitride nanotube suspension;

a metallic matrix-soluble element suspension preparation substep ofmixing the powder of an element soluble in a molten metal of themetallic matrix and an organic solvent to prepare a metallicmatrix-soluble element suspension;

a boron nitride nanotube/metallic matrix-soluble element suspensionpreparation substep of mixing the boron nitride nanotube suspension andthe metallic matrix-soluble element suspension to prepare a boronnitride nanotube/metallic matrix-soluble element suspension; and

an organic solvent elimination substep of eliminating the organicsolvent from the boron nitride nanotube/metallic matrix-soluble elementsuspension to prepare the powder mixture of boron nitride nanotubes anda metallic matrix-soluble element.

ADVANTAGES OF THE INVENTION

According to the invention, there can be provided an Al/BNNT compositewhich has excellent shape flexibility and shape controllability and iscapable of cost reduction. In addition, there can be provided a methodfor manufacturing the Al/BNNT composite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process chart illustrating a method for manufacturing anAl/BNNT composite according to an embodiment of the present invention;

FIG. 2 is a scanning electron microscope (SEM) image of a BNNT/Si powdermixture of Example 1;

FIG. 3 is an SEM image of a cross-sectional view near a surface of anAl/BNNT composite cast article of Comparative Example 1; and

FIG. 4 is an SEM image of a surface of an Al/BNNT composite cast articleof Example 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Initial Study andBasic Concept of Present Invention

From the viewpoints of shape flexibility and shape controllability ofAl/BNNT composites, the present inventors carried out earnest researchon methods for manufacturing Al/BNNT composites by casting. In doing so,the inventors simply mixed an Al melt and BNNTs and subsequentlysolidify the resultant mixture to fabricate casts and examined them. Asa result, it was found that the Al melt had poor wettability with theBNNTs and the solidified Al matrix and the BNNTs easily separated fromeach other (details will be described later).

In the invention, “to wet” means to form a contact angle of 90° orsmaller between the liquid phase (or the solidified liquid phase, i.e.originally liquid phase) and the solid phase (i.e. solid phase allalong). Also, “to be wet” means the state in which the contact interfacebetween the BNNTs and the Al matrix can be observed by electronmicroscopy (e.g. scanning electron microscopy (SEM), or transmissionelectron microscopy (TEM)) and desirably, no unwanted inclusions (e.g.reaction compounds of BNNTs and Al, voids, etc.) are present on theinterface.

The composite of the invention will be hereinafter described with a castas an example.

With an aim to improve the wettability between an Al melt and BNNTs, theinventors formulated a hypothesis as follows. In order to improve thewettability between an Al melt and BNNTs, it would be desired that acomponent with a high affinity for both or at least the Al melt isadded.

Specifically, the inventors have deemed that to an Al melt, a componentthat is readily soluble in the Al melt should added. They have deemedthat presence of an element that is readily soluble in the Al melt(hereinafter referred to as metallic matrix-soluble element, such as Si,Cu, Mg, and Ni) near BNNTs would increase the likelihood of directcontact between the Al melt and the BNNTs with the progress ofdissolution of particles of the element, which could improve thedispersibility of the BNNTs into the Al melt.

As far boron nitride (BN), they have deemed that since BN is a groupIII-V compound and has chemical properties similar to those of carbon(C), which is a group IV element, a group IV element would have a highaffinity for BN (i.e. a group IV element melt would have goodwettability with BN). Based on the above, they have deemed a group IVelement, Si in particular, would be preferable as a component readilysoluble in the Al melt, and addition of Si would improve the wettabilitybetween the Al melt and the BNNTs and the dispersibility of the BNNTs.

In order to confirm this hypothesis, a powder mixture of a BNNT powderand a Si powder was prepared and mixed into an Al melt. The resultantmixture was solidified to fabricate a composite, and this composite wasexamined. As a result, it was confirmed that the wettability between theAl matrix and BNNTs had been improved. The present invention was madebased on this finding.

Preferred embodiments of the present invention will be hereinafterdescribed step by step of the manufacturing process with reference tothe accompanying drawings. However, the invention is not limited to thespecific embodiments described below, and various combinations withknown art and modifications based on known art are possible withoutdeparting from the spirit and the scope of the invention.

[Method for Manufacturing Al/BNNT Composite]

FIG. 1 is a process chart illustrating a method for manufacturing anAl/BNNT composite according to an embodiment of the present invention.As shown in FIG. 1, the method includes a powder mixing step (S1) ofmixing a BNNT powder and a Si powder to prepare a BNNT/Si powdermixture, an alloy melt mixing step (S2) of mixing the BNNT/Si powdermixture and an Al melt to prepare an Al alloy melt mixed with BNNTs, anda casting step (S3) of solidifying the Al alloy melt mixed with BNNTs toobtain an Al/BNNT composite.

The method may also include a remelting and casting step (S4), not shownin FIG. 1, of putting the Al/BNNT composite obtained by the casting stepS3, used as a master ingot, into another Al alloy melt and solidifyingit to obtain an Al/BNNT composite. Also, the casting step S3 and theremelting and casting step S4 may be followed by a shaping step (S5) ofadjusting the outer shape of the Al/BNNT composite according to theneed.

Each step of the method for manufacturing an Al/BNNT composite accordingto the embodiment of the invention will be hereinafter described morespecifically.

(Powder Mixing Step S1)

As described above, this step is a step of mixing a BNNT powder and a Sipowder to prepare a BNNT/Si powder mixture. There is no particularlimitation on the BNNTs used in the invention, and any commerciallyavailable BNNT powder may be used. For example, BNNTs with an averagediameter of 10 nm or smaller and an average length on the order of μmmay be used. Also, the BNNTs are not limited to those with asingle-layer structure, and nanotubes with a multi-layer structure (e.g.2-10 layers) may be used.

In general, in order to mix two powders homogeneously, it is preferablethat they should be similar in average particle size. The powder mixtureof the invention is of a BNNT powder and a Si powder, and as describedabove, the particles of the BNNT powder have a fibrous shape with anaspect ratio (length/diameter) of as large as around 10² to 10³.

The inventors conducted various studies and have found that in the casewhere a BNNT powder and a Si powder are mixed, it is preferable that thespecific surface area (unit: m²/g) should be adopted instead of theaverage particle size as the size selection criterion for the twopowders and that the specific surface area ratio between the BNNT powderand the Si powder should be controlled to be less than 10. The specificsurface area of each powder can be measured by gas adsorption (the BETtheory, the BET method), for example.

For the Si powder to be used in the invention, nanoparticles areeffective. The specific surface area of the Si powder is preferably overone tenth to less than ten times of the specific surface area of theBNNT powder to be mixed into it, and more preferably one third to threetimes. Also, a Si powder of amorphous particles (indeterminate shapeparticles) or scale-like particles (e.g. particles with a thickness ofaround 10 to 30 nm and a diameter of around 50 to 500 nm) is preferablebecause when it is mixed with a BNNT powder, its presence among BNNTs isexpected to reduce entanglement among BNNTs (see FIG. 2 below). Otherthan the above, there is no particular limitation and any commerciallyavailable Si powder may be used.

Moreover, as for a mixing ratio between the BNNT powder and the Sipowder, the ratio between the total surface area of the BNNT powder andthe total surface area of the Si powder is preferably 2 or smaller, andmore preferably 1.5 or smaller. For example, in the case where a BNNTpowder with the average particle size of 4 nm, the average hollowdiameter of 0.84 nm, and the specific surface area of 400 m²/g is mixedwith a Si powder with the average particle size of 10 nm and a specificsurface area of 200 m²/g, it is preferable that the Si powder shouldhave a mass twice that of the BNNT powder so that the total surface areaof the BNNT powder and the total surface area of the Si powder areequal. By setting the mixing ratio in such a manner, aggregation of theBNNT powder can be prevented more effectively.

For mixing the BNNT powder and the Si powder, various mixing methods maybe applied. For example, the powder mixing step S1 may be divided intothe following substeps: a BNNT suspension preparation substep (S1 a), anSi suspension preparation substep (S1 b), a BNNT/Si suspensionpreparation substep (S1 c), and an organic solvent elimination substep(S1 d). The BNNT suspension preparation substep S1 a is a step of mixingthe BNNT powder with an organic solvent to prepare a BNNT suspension,which facilitates disentanglement of the BNNTs. There is no particularlimitation on the organic solvent to be used in the BNNT suspensionpreparation substep S1 a, and alcohols (e.g. methanol, ethanol,1-propanol, 2-propanol, etc.) and ketones (e.g. acetone, methyl ethylketone, and methyl isobutyl ketone, etc.) may be used.

Similarly, the Si suspension preparation substep S1 b is a step ofmixing the Si powder with an organic solvent to prepare a Si suspension,which facilitates disaggregation of the Si powder. There is noparticular limitation on the organic solvent to be used in the Sisuspension preparation substep S1 b, and alcohols (e.g. methanol,ethanol, 1-propanol, 2-propanol, etc.) and ketones (e.g. acetone, methylethyl ketone, and methyl isobutyl ketone, etc.) may be used.

The BNNT/Si suspension preparation substep S1 c is a step of preparing asuspension mixture of the BNNT suspension and the Si suspension(referred to as BNNT/Si suspension). From the viewpoint of homogeneityin the final mixture, the mass ratio of the BNNT powder and the Sipowder in the mixture is preferably within a range from 1:2 to 2:1, andmore preferably within a range from 1:1.5 to 1.5:1.

The organic solvent elimination substep S1 d is a step of eliminatingthe organic solvent from the BNNT/Si suspension to prepare a mixture ofthe BNNT powder and the Si powder (referred to as BNNT/Si powdermixture). There is no particular limitation on the method foreliminating the organic solvent. However, in the case where the amountof the organic solvent in the BNNT/Si suspension is relatively large, amethod for filtering to roughly separate the liquid phase from the solidphase and subsequently drying the solid phase of the BNNT/Si powdermixture may be preferably used, for example.

Although the above description has been made with a BNNT/Si powdermixture as an example, a similar effect is expected to be obtained witha powder mixture of a BNNT powder and a powder of other elements readilysoluble in an Al melt other than Si (e.g. Cu, Mg, and Ni). This isbecause metal powders of these elements have the effect of allowing theAl melt to penetrate into the vicinity of the BNNTs as they dissolve inthe Al melt.

(Alloy Melt Mixing Step S2)

This step is a step of mixing the BNNT/Si powder mixture with an Al meltto prepare an Al alloy melt mixed with BNNTs. There is no particularlimitation on the method for mixing the BNNT/Si powder mixture and theAl melt. However, in order to prevent scattering of the powder mixtureand to make sure that the BNNTs are completely buried in the Al melt, itis preferable that the BNNT/Si powder mixture should be packed in Alfoil or an Al container before it is put into the Al melt.

In the invents on, the Al melt may be a pure Al melt or an Al alloymelt. Herein, pure Al is defined as aluminum with a purity of 99.0% orhigher.

In the case of an Al alloy melt, it preferably has a chemicalcomposition that is capable of forming a eutectic structure. Preferredexamples include aluminum alloys for casting specified by JIS H 5202(e.g. AC1A: Al—Cu alloys, AC1B: Al—Cu—Mg alloys, AC2A and AC2B: Al—Cu—Sialloys, AC3A: Al—Si alloys, AC4A, AC4C and AC4CH: Al—Si—Mg alloys, AC4B:Al—Si—Cu alloys, AC4B and AC8C: Al—Si—Cu—Mg alloys, AC5A: Al—Cu—Ni—Mgalloys, AC7A: Al—Mg alloys, AC8A and AC8B: Al—Si—Cu—Ni—Mg alloys, AC9Aand AC9B: Al—Si—Cu—Mg—Ni alloys). In other words, the Al alloy melt usedin this step includes Al as its main component and at least one of Cu,Mg, Si, and Ni.

As described in JIS H 5202, aluminum alloys for casting may furtherinclude, as trace components, at least one of zinc (Zn), iron (Fe),manganese (Mn), titanium (Ti), lead (Pb), tin (Sn), and chromium (Cr) inaddition to Cu, Mg, Si and/or Ni.

(Casting Step S3)

This step is a step of solidifying the Al alloy melt mixed with BNNTs toobtain an Al/BNNT composite. There is no particular limitation on thecasting method for solidification, and any conventional method may beused.

By performing the steps above, there can be obtained an Al/BNNTcomposite according to an embodiment of the invention.

EXAMPLES

Preferred embodiments of the invention will be hereinafter described inmore detail with examples.

[Experimental 1]

Fabrication of Example 1

According to the manufacturing method described above, a cast article asan Al/BNNT composite (hereinafter referred to as Al/BNNT composite castarticle) of Example 1 was fabricated. First, 1 g of a BNNT powder (withthe average particle diameter of 5 nm and the specific surface area morethan 100 m²/g) was put into 100 mL of ethanol and subjected toultrasonic agitation for one hour to prepare a BNNT suspension (BNNTsuspension preparation step S1 a). The specific surface area wasmeasured with a vapor adsorption amount measuring instrument(BELSORP-maxII, a product of MicrotracBEL Corp.).

Similarly, 1 g of a Si powder (of scale-like particles with the averageparticle thickness of 30 nm, the average particle size of 400 nm, andthe specific surface area more than 100 m²/g) was put into 100 mL ofethanol and subjected to ultrasonic agitation for one hour to prepare aSi suspension (Si suspension preparation step S1 b).

Next, the whole of the BNNT suspension was mixed with whole of the Sisuspension and subjected to further ultrasonic agitation for one hour toprepare a BNNT/Si suspension (BNNT powder of 1 g and Si powder of 1 g,total 200 mL) (BNNT/Si suspension preparation step S1c).

Next, the BNNT/Si suspension was filtered and the solid phase was driedto prepare a BNNT/Si powder mixture (organic solvent elimination step S1d). FIG. 2 is a scanning electron microscope (SEM) image of the BNNT/Sipowder mixture of Example 1. As shown in FIG. 2, it is confirmed thatthe BNNTs 10 and the Si particles 20 are homogeneously mixed. It is alsoobserved that presence of the Si particles 20 has served to reduceentanglement of the BNNTs 10.

Next, the BNNT/Si powder mixture thus prepared was packed Al foil(commercially available) to prepare an Al package. Subsequently, the Alpackage was put into an Al alloy melt (AC4CH: Al-7 mass % Si-0.3 mass %Mg alloy, 1 kg, 700° C.) prepared in a graphite crucible and subjectedto agitation and mixing for one hour. Then, the molten metal was takenout from the furnace together with the crucible to allow the Al alloymelt mixed with BNNTs and Si to solidify by natural cooling to obtain anAl/BNNT composite cast article of Example 1.

Fabrication of Example 2

An Al/BNNT composite cast article of Example 2 was fabricated in thesame manner as Example 1 except that a pure Al melt (A1100, 1 kg, 70.0°C.) was used.

Fabrication f Comparative Example 1

An Al/BNNT composite cast article of Comparative Example 1 wasfabricated in the same manner as Example 1 except that the BNNT powderwas not mixed with an Si powder.

[Experimental 2]

(Observation of Microstructure of Al/BNNT Composite)

For each of the Al/BNNT composite cast articles of Example 1, Example 2and Comparative Example 1, the microstructure of a portion near thesurface of the cast article was observed. FIG. 3 is an SEM image of across-sectional view near a surface of the Al/BNNT composite castarticle of Comparative Example 1. FIG. 4 is an SEM image of a surface ofthe Al/BNNT composite cast article of Example 1.

As shown in FIG. 3, in Comparative Example 1, the BNNTs 10 have peeledoff the Al alloy matrix 30, which suggests that the Al alloy melt didnot wet into the BNNTs sufficiently at the pre-solidification stage. Incontrast, in Example 1, as shown in FIG. 4, the BNNTs 10 are dispersedevenly and mixed well with the Al alloy matrix 30. It is particularlynoteworthy that the BNNTs 10 are inside the Al alloy matrix 30 andintegrated with it. A similar microstructure was also observed withExample 2. This indicates that the Al alloy melt wet the BNNTssufficiently at the pre-solidification stage.

In both Example 1 and Comparative Example 1, AC4CH (Al—Si—Mg alloy),containing Si, was used as the Al alloy melt to which the BNNT powderwas added. Also, in Example 2, the Al melt before the addition of theBNNT/Si powder mixture did not contain Si Considering these, theabove-mentioned clear difference in wettability between the molten metaland the BNNTs cannot be attributed only to presence or absence of Si inthe Al/BNNT composite cast article.

Unfortunately, any detailed mechanism has not been clarified at thepresent stage, but if nothing else, it can be said that by mixing thepowder mixture of the BNNT powder and the Si powder (BNNT/Si powdermixture) into the Al melt, the likelihood of direct contact between theAl melt and the BNNTs was increased with the progress of dissolution ofthe Si powder, which led to an improvement of wettability anddispersibility. As has been described above, the present invention showsan extremely interesting phenomenon.

The above embodiments and experiments are given for the purpose ofdetailed explanation only, and the invention is not intended to includeall configurations of the specific examples described above. Also, apart of an embodiment may be replaced by known art, or added with knownart. That is, a part of an embodiment of the invention may be combinedwith known art and modified based on known art without departing fromthe technical idea of the invention where appropriate.

What is claimed is
 1. A composite of a metallic matrix and boron nitridenanotubes, the metallic matrix comprising aluminum or an aluminum alloy,wherein the boron nitride nanotubes are dispersed in the metallicmatrix, and the metallic matrix is melt-solidified.
 2. The compositeaccording to claim 1, wherein the aluminum alloy comprises aluminum as amain component and at least one of silicon, copper, magnesium, andnickel.
 3. A method for manufacturing a composite of a metallic matrixand boron nitride nanotubes, the metallic matrix comprising aluminum oran aluminum alloy, the method comprising: a powder mixing step of mixinga powder of boron nitride nanotubes and a powder of an element solublein a molten metal of the metallic matrix to prepare a powder mixture ofboron nitride nanotubes and a metallic matrix-soluble element; an alloymelt nixing step of mixing the powder mixture and the molten metal ofthe metallic matrix to prepare a metallic matrix melt mixed with boronnitride nanotubes; and a casting step of solidifying the metallic matrixmelt mixed with boron nitride nanotubes to obtain the composite.
 4. Themethod for manufacturing a composite of a metallic matrix and boronnitride nanotubes according to claim 3, wherein the powder of an elementsoluble in a molten metal of the metallic matrix is a powder of silicon.5. The method for manufacturing a composite of a metallic matrix andboron nitride nanotubes according to claim 3, wherein a ratio betweenthe specific surface area of the powder of boron nitride nanotubes andthe specific surface area of the powder of an element soluble in amolten metal of the metallic matrix is less than
 10. 6. The method formanufacturing a composite of a metallic matrix and boron nitridenanotubes according to claim 3, wherein a mass ratio between the powderof boron nitride nanotubes and the powder of an element soluble in amolten metal of the metallic matrix is equal to or more than 1:2 andequal to or less than 2:1.
 7. The method for manufacturing a compositeof a metallic matrix and boron nitride nanotubes according to claim 3,wherein the aluminum alloy comprises aluminum as a main component and atleast one of silicon, copper, magnesium, and nickel.
 8. The method formanufacturing a composite of a metallic matrix and boron nitridenanotubes according to claim 3, wherein the powder: mixing stepcomprises: a boron nitride nanotube suspension preparation substep ofmixing the powder of boron nitride nanotubes and an organic solvent toprepare a boron nitride nanotube suspension; a metallic matrix-solubleelement suspension preparation substep of mixing the powder of anelement soluble in a molten metal of the metallic matrix and an organicsolvent to prepare a metallic matrix-soluble element suspension; a boronnitride nanotube/metallic matrix-soluble element suspension preparationsubstep of mixing the boron nitride nanotube suspension and the metallicmatrix-soluble element suspension to prepare a boron nitridenanotube/metallic matrix-soluble element suspension; and an organicsolvent elimination substep of eliminating the organic solvent from theboron nitride nanotube/metallic matrix-soluble element suspension toprepare the powder mixture of boron nitride nanotubes and a metallicmatrix-soluble element.